This invention relates to refrigeration systems wherein the flow of refrigerant from the refrigerant condenser to the refrigerant evaporator is controlled by an adjustable refrigerant expansion valve which is responsive to the superheat of the refrigerant to be compressed.
Refrigeration systems are often provided with adjustable refrigerant expansion valves which are controlled in a manner to provide sufficient refrigerant to the evaporator to satisfy the refrigeration load but not enough to allow liquid refrigerant to enter the compressor cylinders, because this condition, known as "slugging" or "flooding", can result in serious damage to the compressor. Prior refrigeration systems have frequently controlled the flow of refrigerant through the expansion valve by sensing the refrigerant pressure or equivalent temperature in the evaporator and the temperature of superheated refrigerant leaving the evaporator. This difference in temperature is known in the art as "superheat".
Prior refrigeration systems have usually been designed to control the refrigerant expansion valve so as to provide a large safety margin of superheat to avoid flooding of the compressor. A large safety margin of superheat is usually necessary because of various operational variables such as changes in head pressure, suction pressure and refrigeration demand, and because of the sluggishness with which refrigeration systems respond to adjustments of the expansion valve and because of the difficulty of accurately measuring the superheat. Because of these variable factors, prior refrigerant control systems tend to provide either overcorrection or undercorrection of the refrigerant flow in the system which in turn can lead either to instability, reduced refrigeration capacity and low efficiency, or possible flooding of the refrigerant compressor.
For example, the refrigerant flow in a refrigeration system tends to be undercorrected during periods of time when the control system detects a high superheat condition but at the same time the superheat is rising rapidly. On the other hand, the flow of refrigerant in the system tends to be overcorrected when the control system detects a high superheat condition but at the same time the superheat is falling due to other system variables. These and other refrigeration system conditions giving rise to instability due to overcorrection or undercorrection, typically occur when condenser fan stages are switched on or off, compressor stages are added or removed, cooling medium flow is changed, or refrigeration load changes rapidly.